Under the 3GPP standards, a NodeB (or an eNB in Long Term Evolution (LTE) standards) is a base station via which mobile devices connect to the core network. Such base stations are designed to cover a relatively large area and are often referred to as ‘macro’ base stations to distinguish them from base stations designed to cover a smaller area.
The 3GPP standards body has also adopted an official architecture and started work on a standard for so called ‘home’ base stations to provide short range 3G radiofrequency (RF) coverage. Where the home base station is operating to provide a UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network (UTRAN) type cell, the home base station is sometimes referred to as an HNB. Where the home base station is operating in accordance with the (Long Term Evolution) LTE standards to provide an LTE or evolved UTRAN (E-UTRAN) cell, the home base station is sometimes referred to as an HeNB. A similar architecture will also be applied in the WiMAX network. In this case, the home base station is commonly referred to as a femto cell. For simplicity, the present application will use the term HeNB to refer to any LTE home base station, the term HNB to refer to UMTS/UTRAN home base stations, and the term home base station generically to refer to HNB, HeNBs or other such base stations. Each home base station will provide radio coverage (for example, 3G/4G/WiMAX) within the home, small and medium enterprise, shopping Malls etc. and will connect to the core network via a public or corporate broadband access network (for example via an ADSL link to the Internet) and in the case of the 3GPP standards, via an optional home base station gateway (HNB-GW) which typically will aggregate traffic from several home base stations. The home base station may also connect to the access network via a suitable residential gateway (RGW).
The functionality of a home base station is often provided by a so called ‘femto access point’ (FAPs) and the term ‘home base station’ and ‘femto access point’ are often used interchangeably. An FAP can comprise a single mode FAP which provides the functionality of a single home base station operating a cell of a particular radio access technology (e.g. UTRAN or E-UTRAN). An FAP can also comprise a dual mode (or possibly multimode) FAP which provides the functionality of two (or possibly more) home base stations each effectively operating a cell of a different radio access technology (e.g. a UTRAN cell and an E-UTRAN cell).
In order to provide for mobility from one cell of the communications network to another, each FAP has a network monitor mode (NMM) functionality that allows it to scan for neighbouring cells and to place information, such as cell IDs or the like, relating to the neighbouring cells detected during the scan into a neighbour table or database at the FAP.
Typically, when initiating a call, user equipment (UE) such as a mobile telephone (MT) or other mobile communications device (MCD) will register either with a home base station or a macro base station based on, for example, the ability of that base station to support the call relative to other base stations in the vicinity. Once the call is initiated, it sometimes becomes necessary to relocate the call to another base station, using a ‘mobility’ procedure, due to a change in the relative ability of the original base station to continue to support the call, for example as a result of a change in the location of the mobile communication device or other factors which affect the quality of service offered by the original base station.
Each home base station may be configured to operate using one of a plurality of access modes, namely: ‘closed’ in which the home base station operates as a closed subscriber group (CSG) cell; ‘hybrid’ in which the home base station operates as a CSG cell where at the same time, non-CSG members are allowed access (for example, to allow preferential treatment of CSG members); and ‘open’ in which the home base station operates as a normal (non-CSG) cell.
Referring to FIG. 5, which is a timing diagram for a typical handover procedure, when a mobility procedure is initiated to handover/redirect a mobile communication device from one base station (a ‘source’ home base station in the example) to a second ‘target’ home base station a check will be made, at the target home base station, as to whether sufficient resources are available to support the mobile communication device. When the mobility procedure is initiated, access control and/or membership verification may also be performed in dependence on the access mode of the target home base station. In the case of a UMTS/UTRAN access control and/or membership verification may be performed by the HNB-GW, the target HNB and/or the core network. In the case of LTE/E-UTRAN access control and/or membership verification may be performed by a mobility management entity (MME) and/or a target HeNB.
The resource checking and access control/membership verification procedures can be relatively time consuming and can therefore cause an undesirable delay before the mobile communication device can be used for communication purposes. Moreover, if insufficient communication resources are available to support the mobile communication device and/or access control/membership verification fails then the mobility procedure is rejected and a failure indication is propagated back to the source home base station which then has to attempt handover of the mobile communication device to another base station (home or macro). This results in further delay and additional energy consumption.